Pulse detector system



Sept. 12, 1950 w. H. FORSTER PULSE DETECTOR SYSTEM 2 Sheets-Sheet 1 W Rm mi Em m H A M w N\ Sg t. 12, 1950 w. H. FORS'TER 2,522,110

PULSE DETECTOR SYSTEM Filed Dec. 21, 1944 2 Sheets-Sheet 2 HH 5' NW N Mn I N VEN TOR. W /Iqm h. Ear-afar BY 09 6M1 A T TOF? NE Y5.

Patented Sept. 12, 1950 OFFICE PULSE DETECTOR. SYSTEM WilliamiixForster, Philadelphia, Pa, assi'ghor, "bymesne assignments, to PhiicoCorporation, Philadelphia, Pa; a corporation of Pennsyl- U Vania 1Application December 21 1944,; Serial No.

'I'Glaimn. (-Cl. ZED-27 The present invention relates to electrical orparatus and particularly to a radio "receiver. More particularlystilltheinvention relates to an improved detector system for thereception of pulse repetition frequency modulatedsi'gnals.

' The system "of the'instant'invention; in its preferred embodiment,utilizes a non-oscillating multivibratoras'an important element of theus tector and combines therewith a p'ulsewitlth clis criminator.

--In my present invention the "receivedsignal is applied to a limiterand thecutput thereof is passed through apulse width discriminator,thereby reducing pulse interference, after which the "output of thediscriminator is applied to a multlvibrator and causesfiring thereof.The output pulse of the 'multivibrator is' integrated; the resultantoutput from the integrator being a sawtooth wave form containing theoriginal modulation, which is thereafter separated from other componentsby alo'w pass filter'leaving only the audiosignal.

'Aswill be clear from the above, it is perhaps more proper to term thedetector oneof pulse intervals rather than-a pulse repetition frequencydetector since thepeak amplitude of the saw tooth wave to which theaudio'slgnal is proportional is, in reality, a linear function-of theinterval between successive pulses. However, for the purposes or thisdescription; it'is believed-sub ficiently accurate to speak of thedetector as -a pulse repetition frequency detector.

"As i-l l-ustrativeof the situation-mentioned in the precedingparagraph, a rekc. pulse repetition frequency with -a- 2 deviation plusor minus will change the. intervals between pulses from L00microseconds, center frequency,- down to 83 thetical tube with acharacteristic.sapproaching anshyperbola, the output of the detectorwill he distortionless. v I u =.The present system was primarilydesigned for military communications and in the system,

therefore, a distortion of from 10 to 20% is without significance.

distortion in r the detector. if the transmitter is suitably'rnodulated. Thus, if the transmitter utilizes a hypo- .It is anobjectsof the present invention to pro '55 vide. an efficient detectorfor pulse repetition fre quenc'y modulated signals. It is another"object of the invention to pro vide such a detector in which means areprovided for eliminating pulse interferences.

It is a further object of the invention to provide such a detectorhaving a high signal tc noise ratio. I Other objects and features of theinvention "will appear when the followingtlescription is consld ered inconnection with the ap ended drawings in which--- Figure l is aschematic diagram of the detector systemofrny invention;

Figure 2 is a curve showing the transmitted signal;

FigureBis a curve showing the video frequen cy signal input to thelimiter stage of the detectorsystem;

Figure 4 is avoltage curve showing the output of the limiterasmodifiedby the discriminator action thereof Figure '5 isa voltage curveshowing the ouput signal from the rnultivibrator; and

Figure 6 is a voltagecurve showing the output signaPTrom theintegratontube and in dashed line the audiooutputof the system.

Referring now to Figure 1, there] is shown therein at l0 a diodedetectorwhich follows the last intermediate frequency-amplifier stageand applies pulses to the control grid of the pentode II which pentodeserves as'a limiter in its grid circuit and as an integrator in itsplate circuit. The plate circuit time constant (i; e.- the values oi theresistance 1-2 and capacity l3) are so actju'sted in connection with thefollowing pentode Hi thatwwhen the transmitted pulses are of a length ofthree-microseconds and the mean interval between pulses 100microseconds, an inter-- fering pulse of'overload amplitude will notintegrate to-a sufiicient-voltage to cause the Dentode 14, which isapich-oif tube; todraw current.

Referring to Figure 3, it will be seen that the inputto "the pentode Hcontains interfering pulsessuch asthose designated H: which are 'I heoutputofthe pento'de I4 is applied to a multibrator comprising tubes 16and i7. The multivibratoris of normal type and is adjusted(to"bench-oscillating; Therefore when a-signal pulse such as indicatedat I8 in Figure 4 is applied to the multivibrator, it goes through asingle cycle and produces in its plate circuit a square topped impulsesuch as indicated at I9 in Figure 5. These impulses are applied throughthe capacity coupling 20 to the pentode 2| which pentode is biasedbeyond the cut-off point. The applicationof the wave of Figure to theinput grid of the pentode 2| causes this pentode to be operatedalternately above and below plate cur: rent cut-oii values. Thus duringthe period of an impulse such as I9 the plate resistance of the pentode2I is very low and any charge which may have accumulated in the platecircuit capacity, including the capacity of the tube itself as well asthe capacity of the shunted condenser 22, or in some instances theinherent internal capacity of tube 2| alone, will leak off to groundthrough the pentode.

During the remainder of the cycle, that is during the interval betweenpulses I9 of Figure 5, the plate resistance of the pentode 2| isextremely high and the condenser 22 is charged by the plate sourcethrough the plate load resistor 23. The resultant signal output from thepentode is illustrated in Figure 6. It will be seen from this figurethat the amplitude of the signal varies in direct proportion to thespacing between pulses I9 since during the charging intervals thevoltage across the condenser 22 increases linearly (or more preciselyexponentially) and, therefore, the maximum potential reached by thecondenser during a particular charging interval is proportional to thelength of time that the pentode 2| remains below plate current cut-offvalue.

The signals of Figure 6 are applied to the control grid of the triode 24and the amplified signals therefrom are passed through the audio filter25 and thence to the audio stages of the receiver.

Although the above has indicated the mode of operation of the device itwill perhaps be clearer if a detailed description of that operation isgiven. The operation of the device is as follows:

The circuit of diode I0 acts as a source of demodulated I. F. andsupplies pulse signals.

Tube II is normally conducting to a moderate extent because of the zerobias on its grid and its low; plate and screen voltages. The inputpulses from detector I0 (see Fig. 3) drive tube II to cutoff-,because ofits small grid voltage range. Tube, ||;thusserves as a limiter forlimiting signals from pulse source I0.

,When tube II is cut on, the B supply starts to charge condenser I3through resistor l2. Condenser I3 thus integrates the energy from the Bsupply source. The time constant of the circuit comprising resistance I2and condenser I3 is of the order of the length of a typical pulse, i.e., 3 microseconds. Condenser I3 discharges instantly when tube I I isconducting, and it is thus inactivated as an integrator and restored toits initial condition in response to the presence ofa signal-in theinput of the tube II.

Tube I4 is overbiased, to the extent that it will only respond when apulse signal has cut off tube II, and integration in .condenser I3 hascontinued for about 3 microseconds. When tube I4 conducts it appliesanegative pulse to the grid of tube I1 (and incidentally on the plate oftube t (See Fig. 4.)

The negative pulse on the grid of tube I1 cuts to the grid of the tubeIt, where it appears across its grid leak. The condenser between the.

plate of tube I1 and the grid of tube I6 does not have time to dischargethrough its shunt resistor during this operation, and substantially theentire positive pulse is applied across the grid leak of tube I6. (TheR. C. circuit in the common lead between the cathodes of tubes I6 and I1and ground has a long time constant, andmaintains the cathodes at afixed-potential, substantially as a battery would.) Tube I6 nowconducts, and its plate current, passing through its plate loadresistor, holds its plate potential down to a value of the order of thatto which it was driven by the pulse from tube Id. The condenser betweenthe plate of tube It and the grid of tube IT is now gradually chargedfrom the B supply, through the grid leak of tube H, to a potential equalto the plate current drop in the plate load resistor of tube I6. As thevoltage across the condenser rises, the current through the gridresistor of tube II falls, and the grid potential of tube I1 risesgradually until it passes the cutofi potential. When this happens, tubeI'I conducts, and its plate current fiow drops its plate potential. Thenegative pulse thus generated appears on the grid of tube It and cutsofi. the now of its plate current, leaving the multivibrator incondition to be triggered again. The plate load of tube I1 is tapped totransmit its output to condenser 20. (See Fig. 5.)

Thus, the full voltage available at the tap on the voltage divider 'isapplied to the condenser 20 for the duration of the multivibrator pulse.Tube 2| is thus rendered conductive for this period, becomingnon-conductive again when the multivibrator restores itself to itsinitial condition, dropping the voltage at the tap point. As soon astube 2| starts conducting, condenser 22 discharges, and practically thewhole B supply voltage is dropped in resistor 23. During thenon-conducting period of tube 2|, that is, in the interval betweenmultivibrator pulses, condenser 22 charges up substantially linearlythrough resistor23. (See Fig. 6.) The resulting triangular signals aresupplied to the grid of cathodeloaded amplifier 24, filtered in the lowpass audio filter 25, and delivered to the output terminals (see Fig. 6,dashed line).

If an interfering pulse shorter than a signal pulse is applied so as tocut ofi tube II, condenser I3 does not charge up enough to fire tube I4,and condenser I3 is instantly discharged at the end of the interferingpulse. If an interfering pulse is long enough to fire tube I4, it willnot actuate or otherwise affect the multivibrator while themultivibrator is forming a pulse. If

the multivibrator is not forming a pulse when,

tube I4 fires, it forms a false pulse which will actuate tube 2 I. Tube2| will then conduct, and condenser 22 will instantly discharge. Underthese conditions condenser 22 will fail to charge up to the full valuecorresponding to the full inter-pulse time interval because before itgets to that value it will be discharged, and it will start,

to charge again following the false pulse. It will again be prematurelydischarged by the next true pulse, and the two signals of reduced inlateafter the last previous true pulse that the multivibrator is-fired bythe false pulse, and

therefore fails to respond to the next true mulsea In this case tube 2|is cut off for morethanthe normalninter-pulsev time, and c'ondenser 22.is charged (exponentially) to an unduly high value. From the above.description of the circuit and its operation, it will be clear thatthesignal pulse must be sufficiently higher than the-peak noise level tofire the pick-off tube or pentode l4 and also clearthat anynoiseintroduced into the output results only from variations in .the firingtime of the ml11tiVlb1at0Il6'-.-'l1. Any such noise canbe reduced to aminimum by utilizing very short signal pulses In practice pulses of fromone-*halfto one microsecond in duration have been successfully used.Likewise, interference dueto thermal noise affects only the firing timeof the multivibrator. Hence, the shorter the signal pulse the less thepossible variation in firing time and the less the thermal noise.Limiting factors are methods of obtaining short pulses and the increasedintermediate frequency bandwidths required for the shorter pulses. Asbefore, in practice pulses from one-half to one microsecond have beensuccessfully used, the onehalf microsecond pulse being about the lowestlimit which can be secured.

,In connection with the shortness of the pulse it may be mentioned thatradar interference will affect the fi ring time of the multivibratoronly when the radar pulse coincides w ith the signal pulse.fIhechancesof this occurring for a system in which the duty cycleishigh, say on the order of 100 mmore, are approximately 1:100 orwhatever the duty cycle happens to be. H

The fidelity of a detector system such as the one of my invention islimited only by thecenter valueof the pulse repetition frequency. Ingeneral, the pulse repetition frequency must be at least twoand one-halftimes the maximum modulating frequency, or more specifically,theminipulse repetition center frequency minus the maximum negativedeviation in frequency must be sufficiently greater than twice themaximum modulating frequency so that the low pass filter will remove thepulse repetition frequency components in the integrated sawtooth waveand still prevent speech inversion.

Thus, for fidelity comparable to that provided by commercial telephoneservices (for example, 200 to 2500 cycles per second) center frequenciesof from 7 to 20 kc. have been satisfactorily used. For higher fidelitiesthe center pulse repetition frequency must be increased, maintaining thevalue at approximately two and one-half times the maximum modulationfrequency.

The characteristics of the detector system described are such that thesignal-to-noise ratio changes from O to over 100 upon a 3 decibel changein signal level. The pulse repetition frequency system has a thresholdsensitivity determined by that signal level necessary to produce a pulseheight about twice the root-mean square value of the thermal noise. Anadditional 3db of signal locks the detector to the signal and produces asignal-to-noise ratio of over 100 which remains constant for all highersignal levels.

In order to discriminate between pulses on the basis of Width alone, itis necessary to limit the signal pulse amplitude at the threshold value.'f

Of course, this sets the signal-to-noise ratio for all signal levels butby sacrificing a few decibels at threshold sensitivity it is possible toraise the signal-to-noise ratio to several hundred for all signal levelsabove the new threshold. Consquent-lyn-iti's possible with the systemtop'r'o duce veryh'igh signal to noise levels such as re sultwithordinary 'ampl-itlude' modulated and frequency modulated systems.Ihavefound, how ever; thatfor the system above described and for allpractical purposes, consideringthe low fidelity requirements, asi'ghalto-n'oise ratio of about lsadequate. a 1 w Be'cause.of the limitingaction :of, the pen'tode H, as heretofore described, together with theoverload characteristics of the intermediatefre quency.amplifier,'thereis no necessity for automatic volume control. The intermediate frequencycircuits must, however, be designed with very short grid time constantsin order to prevent the intermediatefrequency amplifier from iback' i-ngrofff in the presence of high overload signals,- Whether signal orinterference.

,i I nere are many possible ways of obtainingthe firingcharacteristicsof tire non oscillatingmulti vibrator used i'nfthe system and describedhereina'bove. For example, thyratrons could be utilized together withcircuits similar to those of conventional sawtooth oscillators, orblocking oscillators might be employed. In addition, there area numberof integrator circuits which might be utilized-and, similarly, a numberof pulse width discriminators.

Thus, while I have described a preferred embodiment of my invention, Ido not desire to be limited to the description which is given hereinsolely for purposes of illustration, but rather to be limited only bythe appended claims.

What is claimed 1. In a detector of pulserepetitionfrequency modulatedsignals, in combination, means for limiting the amplitude of receivedfrequency modulated signals, means for eliminating signals of lessthanapredetermined duration, means for producing equal intervaleq-ualamplitude signals spaced in accordance with received signals exceedingthe predetermined duration, and means for integrating said spaced equalinterval signals to produce signals containing the original modulationof said pulse repetition frequency modulated signals.

2. In a detector of pulse repetition frequency modulated signals, incombination, means for limiting the amplitude of received frequencymodulated signals, means for eliminating signals of less thanpredetermined duration, means for producing equal interval equalamplitude signals spaced in accordance with received signa1s exceedingthe predetermined duration, means for integrating said spaced equalinterval signals to produce signals containing the modulation of saidpulse repetition frequency modulated signals, and a filter system forpassing only the modulation components of said last mentioned signals.

3. In a detector of pulse repetition frequency modulated signals, incombination, means for limiting the amplitude of received frequencymodulated signa1s, means for eliminating signal of less thanpredetermined duration, a multivibrator for producing equal intervalequal amplitude signals, said multivibrator being fired in accordancewith the received signals exceeding the predetermined duration, andmeans for integrating said output signals from said multivibrator toproduce signals containing the original modulation of said pulserepetition frequency modulated signals.

4. In a detector of pulse repetition frequency modulated signals, incombination, means for limiting the amplitude of received frequencymodulated signals, means for eliminating signais of'iess thanpredetermined duration, a multivibrator for producing equal intervalequal amplitude signals, said multivibrator being fired in accordancewith the received signals exceeding the predetermined duration, and apentode biased to cut off said output from said multivibrator beingapplied to said pentode, said pentode integrating said multivibratoroutput signals and producing signals containing the original modulationof said pulse repetition frequency modulated signals.

5, In a detector of pulse repetition frequency modulated signals, incombination, means for limiting the amplitude of received frequencymodulated signals, means for eliminating signals of less thanpredetermined duration, a multivibrator for producing equal intervalequal amplitude signals, said multivibrator being fired in accordancewith the received signals exceeding the predetermined duration, a pentodbiased to cut 011, said output from said multivibrator being applied tosaid pentode, said pentode integrating said multivibrator output signalsand producing signals containing the original modulation of said pulserepetition frequency modulated signals, and a filter system for passingonly the modulation components of said last mentioned signals.

.6. In a detector of pulse repetition frequency modulated signals, incombination, a pentode tube having its input circuit adjusted to limitthe amplitude of received frequency modulated signals and its platecircuit adjusted to eliminate signals of less than predeterminedduration, a multivibrator adjusted to go through a single cycle whenfired, a pick-01f tub in the output circuit of said pentode, saidpick-off tube serving to apply output signals from said pentode to saidmultivibrator, and means in the output circuit .of said multivibrator tointegrate the output 40 thereof toproducesignals containing the originalmodulation. i w ,1, 7.- In a detector of pulse repetition frequencymodulated signals, in combination, a pentode tube having its inputcircuit adjusted to limit the amplitude of received frequency modulatedsignals and itsplate circuit adjusted to eliminatesignals of less thanpredetermined duration, a multivibrator adjusted to go through a singlecycle when fired, a pick-011 tube in the output circuit of'said pentode,said pick-off tube'serving to apply output signals from said pentode tosaid multivibrator, means in the output circuit of said multivibrator tointegrat the output thereof to produce signals containing the originalmodulation, and a filter system for passing only the modulationcomponents of said last mentioned signals.

. WILLIAM H. FORSTER.

REFERENCES CITED The following references are of record in the file ofthis patent:

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